Infrared Spectroscopy of Gas Phase Benzenium Ions: Protonated Benzene and Protonated Toluene, from 750 to 3400 cm^–1

Abstract

Gas phase C 6H 7 (+) and C 7H 9 (+) ions are studied with infrared photodissociation spectroscopy (IRPD) and the method of rare gas tagging. The ions are produced in a pulsed electric discharge supersonic expansion source from benzene or toluene precursors. We observe exclusively the formation of either the C 2 v benzenium ion (protonated benzene) or the para isomer of the toluenium ion (protonated toluene). The infrared spectral signatures associated with each ion are established between 750 and 3400 cm (-1). Comparing the gas phase spectrum of the benzenium ion to the spectrum obtained in a superacid matrix [ Perkampus, H. H.; Baumgarten, E. Angew. Chem. Int. Ed. 1964, 3, 776 ], we find that the C 2 v structure of the gas phase species is minimally affected by the matrix environment. An intense band near 1610 cm (-1) is observed for both ions and is indicative of the allylic pi-electron density associated with the six membered ring in these systems. This spectral signature, also observed for alkyl substituted benzenium ions and protonated naphthalene, compares favorably with the interstellar, unidentified infrared emission band near 6.2 microm (1613 cm (-1)).

Laboratory Spectroscopy of Protonated PAH Molecules Relevant For Interstellar Chemistry

“…Subsequent IRMPD spectra reported for isolated C 6 H + 7 in the fingerprint range (Fig. 1) using CLIO and monitoring H 2 loss (Jones et al 2003;Dopfer et al 2005) and higher-resolution IRPD spectra of Ar-tagged C 6 H + 7 ions (Douberly et al 2008) (Dopfer 2005;Lorenz et al 2007;Dopfer 2008;Knorke et al 2009;Zhao et al 2009); the spectra of benzene H + and naphthalene H + were recorded using CLIO, whereas all other spectra were obtained using FELIX; bands marked by asterisks are largely suppressed due to reduced laser power. Right: IR spectra of coronene, coronene + and coronene H + compared to an UIR spectrum (Knorke et al 2009).…”

“…Ion and cluster ion generation has been accomplished in an electron-impact or dischargedriven supersonic plasma expansion. This approach was used to produce protonated benzene and naphthalene, C 6 H + 7 and C 10 H + 9 , and their weakly-bound clusters with ligands L = Ar, N 2 , CH 4 , and H 2 O (Solca & Dopfer 2002;Solca & Dopfer 2003b;Douberly et al 2008;Ricks et al 2009). Alternatively, isolated H + PAH can be generated by chemical ionization of PAH in an ion cyclotron resonance (ICR) mass spectrometer using protonating agents, such as CH + 5 or C 2 H + 5 , as successfully demonstrated for C 6 H + 7 and C 10 H + 9 (Jones et al 2003;Dopfer et al 2005;Lorenz et al 2007).…”

“…Two major spectroscopic strategies have successfully been employed to obtain IR spectra of isolated and microsolvated H + PAH ions. The first technique couples modern low-intensity optical parametric oscillator (OPO) laser systems, typically operating in the 1000-4000 cm −1 range, with tandem mass spectrometers to record single-photon IRPD spectra of cold H + PAH-L m cluster ions by monitoring the loss of weakly-bound ligands L (Solca & Dopfer 2002;Solca & Dopfer 2003b;Douberly et al 2008;Ricks et al 2009). The low intensities of OPO lasers are usually insufficient to drive multiple-photon processes.…”

Laboratory infrared spectroscopy of PAHs

“…The group of Duncan recently recorded IR spectra of protonated benzene (Douberly et al 2008) and naphthalene (Ricks et al 2009) using an optical parametric oscillator (OPO) source that provides mJ pulse energies down to wavelengths of about 10 µm. Argon tagged protonated molecules are produced in the expansion from a pulsed discharge nozzle and mass-selected in the first arm of a reflectron TOF mass spectrometer.…”

Gas-phase infrared spectra of cationized nitrogen-substituted polycyclic aromatic hydrocarbons

“…Indeed, because of the high proton affinity of these species, it is not unlikely that they occur in their protonated forms. Recent experiments involving ion spectroscopy of various protonated PAH species report absorption bands very close to the 6.2 μm interstellar band (Douberly et al 2008;Knorke et al 2009;Lorenz et al 2007;Ricks et al 2009;Vala et al 2009a,b), although a detailed comparison between all protonated PAH spectral features and the interstellar UIR bands has not been performed to date. These protonated PAHs also have a closed-shell electronic structure.…”